The invention relates to a semiconductor device of the RESURF type with a low-side lateral DMOST (LDMOST), comprising a semiconductor body of predominantly a first conductivity type and a surface region which adjoins a surface, which is of a second conductivity type opposed to the first, and which forms a first p-n junction with the semiconductor body at the side facing away from the surface, which LDMOST comprises a back gate region in the form of a surface zone of the first conductivity type provided in the surface region, with a source region in the form of a surface zone of the second conductivity type in the back gate region, and a channel region defined between the source region and an edge of the back gate region, and a drain region in the form of a surface zone of the second conductivity type situated at a distance from the back gate region, while a separation region of the first conductivity type is provided in the surface region around the LDMOST so as to adjoin the surface and extend towards the semiconductor body, and one or several breakdown voltage raising zones of the first conductivity type are provided between the drain region and the back gate region and between the drain region and the separation region, which breakdown voltage raising zones adjoin the surface, this surface being provided with an insulating layer on which a conductor track is provided which is connected to the drain region and which extends over the voltage raising zones and the separation region.
The LDMOST is intended for a so-called low-side application, i.e. during operation the drain region may be at a comparatively high voltage relative to the semiconductor body and the back gate/source region, while the back gate/source region carries a comparatively low voltage relative to the semiconductor body. No breakdown voltage raising zones are present then between the back gate region and the separation region in practice.
The article "A Versatile 700-1200-V IC Process for Analog and Switching Applications" from "IEEE Trans. on Electron Devices, vol. 38, no. 7, Jul. 1991, pp. 1582-1589) discloses a device of the kind mentioned in the opening paragraph which is particularly suitable as a switching element for high voltages. An n-channel LDMOST lies in the surface region in the known device. The surface region is formed by an n-type epitaxial layer on a semiconductor body formed by a p-type semiconductor substrate. A p-type back gate and n-type source and drain regions are provided in the surface region. Gate oxide is present on the surface above the channel region. Electrical conductor tracks are provided on the source region, the back gate region and the gate oxide, forming a source/back gate connection and a gate electrode. The source and back gate regions are short-circuited. To render the semiconductor device suitable for high voltages, the so-called RESURF principle is used, i.e. the net doping of the surface region in atoms per unit surface area is so small that, when a voltage is applied across the first p-n junction, the surface region is depleted throughout its thickness at least locally before breakdown occurs. As a pilot value for the net doping, approximately 1.times.10.sup.12 atoms/cm.sup.2 is taken in the case of RESURF. In the known semiconductor device, one or several breakdown voltage raising zones are provided between the back gate and drain regions and between the drain region and the separation region, these zones taking the form of one or several rings extending around the drain region. The breakdown voltage raising zones ensure that, given a high voltage at the drain connection relative to the back gate and the substrate, the surface region is depleted not only from the first p-n junction between substrate and epitaxial layer, but also from p-n junctions between the breakdown voltage raising zones and the epitaxial layer. The epitaxial layer is thus depleted from several sides, so that even at a doping concentration of the surface region higher than approximately 1.times.10.sup.12 atoms/cm.sup.2, for example approximately 1.5.times.10.sup.12 atoms/cm.sup.2, the RESURF condition can be fulfilled: the epitaxial layer can be completely depleted at least locally before avalanche breakdown occurs between substrate and back gate. The breakdown voltage raising zones here spread the electric field in the surface region, so that no high electric fields occur locally. The channel region is present at the surface below the gate oxide. Charge carriers from the channel region must pass below the breakdown voltage raising zones through the so-called drift region to the drain. A breakdown voltage raising zone which lies too close to the back gate region blocks the charge carriers coming from the channel region. This is why in an LDMOST a region between back gate and drain adjoining the surface and the back gate region is kept free from breakdown voltage raising zones.
When the voltage at the drain region increases in an n-channel LDMOST after switching-off of the LDMOST, the breakdown voltage raising zones will follow the drain region as regards voltage until punch-through to the p-n junction between back gate and surface region charges the breakdown voltage raising zones negatively and said zones are depleted at least partly. When the drain voltage decreases now after switching-on of the LDMOST, the breakdown voltage raising zones cannot be discharged through the blocked p-n junctions and accordingly remain negatively charged for some time, which is accompanied by a high ON-resistance (resistance between drain and source) of the LDMOST because a portion of the surface region between back gate and drain, i.e. the drift region, remains partly depleted. This high ON-resistance remains until holes are supplied from the p-n junction between the back gate and the surface region, for example, by leakage or punch-through.